The present invention relates to wavelength-selective polarization conversion elements which convert non-polarized light into light with a polarization direction according to a wavelength region (or color) and image projection apparatuses equipped with the element, such as liquid crystal projectors.
Japanese Patent Laid-Open Application Nos. 2001-154152 and 2000-19455 have disclosed image projection apparatuses which perform color-separation and color-combination by a polarization beam splitter. In the apparatuses, non-polarized light emitted from a light source is divided into plural luminous fluxes by a lens array, secondary light source images are formed by the divided luminous fluxes, and luminous fluxes from the secondary images of the light source are overlapped with each other on an image-forming element such as a liquid crystal panel, thereby illuminating the image-forming element with light with substantially even luminance.
Each divided luminous flux from the lens array enters a polarization conversion cell corresponding to each lens cell of the lens array, plural polarization conversion cells being provided in a polarization conversion element. Each polarization conversion cell has a polarization beam splitting film, a half-wave plate and a reflecting surface. The non-polarized light that entered the polarization conversion cell is separated into P-polarized light and S-polarized light by the polarization beam splitting film. The P-polarized light is transmitted through the polarization beam splitting film. The polarization direction of the P-polarized light is rotated by 90 degrees by the half-wave plate, and the resulting S-polarized light emerges from the polarization conversion element.
On the other hand, the S-polarized light is reflected by the polarization beam splitting film, and then reflected by the reflecting surface to emerge from the polarization conversion element. The S-polarized light with a uniformed polarization direction, which emerged from the polarization conversion element, enters a condenser lens.
The S-polarized light that emerged from the condenser lens is separated into light of first and second wavelength regions and light of a third wavelength region by a dichroic element. In this stage, the light of the first wavelength region and the light of the second wavelength region travel in the same optical path and have the same polarization direction. Then, the light of the first and second wavelength regions is passed through a wavelength (color)-selective phase plate. This is for introducing light of the first wavelength region and light of the second wavelength region to first and second image-forming elements, respectively, by a polarization beam splitter.
Thereby, it is possible to separate the light of the first and second wavelength regions into light components with different polarization directions.
The wavelength-selective phase plate is made by laminating a plurality of stretched films. It has a characteristic which converts only one of the light of the first wavelength region and the light of the second wavelength region into light with a polarization direction orthogonal to its original polarization direction and transmits the other one without changing its polarization direction.
As described above, image projection apparatuses which perform color-separation by using a polarization beam splitter generally include a polarization conversion element located near a lens array and a wavelength-selective phase plate located near the polarization beam splitter.
Japanese Patent Laid-Open Application Nos. 2000-19455 and H11(1999)-153774 have disclosed image projection apparatuses which use a particular optical element. The apparatus disclosed in Japanese Patent Laid-Open Application No. 2000-19455 uses an optical element constituted by a first dichroic layer transmitting light of a predetermined wavelength region and reflecting light of another wavelength region, a phase layer rotating the polarization plane of light transmitted through the first dichroic layer by 90 degrees and a total reflection layer totally reflecting light from the phase layer. The apparatus disclosed in Japanese Patent Laid-Open Application No. H11(1999)-153774 uses a polarization beam splitter having wavelength selectivity and performing polarizing, analyzing, color-separation and color-combination.
However, using the wavelength-selective phase plate separate from the polarization conversion element as disclosed in Japanese Patent Laid-Open Application No. 2001-154152 increases the number of optical elements constituting the image projection apparatus. In addition, the configuration becomes complex since holding and positioning of the wavelength-selective phase plate and a cooling structure for cooling the wavelength-selective phase plate made of a multilayer film are necessary.